<!doctype html>
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1902540">
<script>
let fuzz = () => {
  document.body.appendChild(document.createElement("iframe"))
  document.getElementById("b").appendChild(document.createElement("plaintext"))
  document.documentElement.offsetHeight;
  window.frames[0].document.body.appendChild(document.getElementById("a"))
  window.frames[0].document.body.appendChild(document.getElementById("c"))
}
</script>
<style>
:last-child {
  display: ruby-text-container;
}
</style>
<body onload="fuzz()">
<fieldset id="a"></fieldset>
<ruby id="b">
<ul id="c"></ul>
</ruby>
</body>

